Hydroponic NFT system for 36 plants, 80 mm channels

The 200 mm spacing between centres gives each plant room to open out without touching its neighbour, which matches the spacing of around twenty centimetres recommended for leafy vegetables grown to full size. Your headed lettuces therefore reach their adult size here, whereas a tighter spacing forces you to cut them young. Remove the runners from strawberry plants as they appear, since they cannot root in a channel and they draw on part of the plant's reserves.

The 64-litre tank, measuring 60 x 40 x 32 cm, holds almost the entire volume of the circuit, since the channels retain only a thin film. The submersible pump, a Hailea HR1000 with a nominal flow rate of 1,050 L/h, feeds the four channels through a four-outlet line, and the water returns to the tank via drainage caps and 32 mm push-fit fittings. The caps are glued at the factory at both ends of each channel, so all you have to do is connect the supply and the return.

In summer, the benefit shows mainly in the afternoon, when the air in a greenhouse or conservatory heats up faster than the solution protected by the air gap. The reservoir, however, has no double wall: place it in the shade, since it holds almost all the water in the circuit and sets its starting temperature. A thermometer placed in the reservoir at the end of the day tells you whether you are staying below the 20 °C mark, or whether you need more shading.

You dose the 64-litre reservoir by multiplying the per-litre dose. During full growth, TriPart is added at 1.8 ml of Grow, 1.2 ml of Micro and 0.6 ml of Bloom per litre, which comes to roughly 115 ml, 77 ml and 38 ml for a full tank, with a conductivity of 1.3 to 1.8 mS/cm. When the strawberry plants flower, the proportions are reversed: 0.8 ml of Grow, 1.6 ml of Micro and 2.4 ml of Bloom per litre, which comes to roughly 51, 102 and 154 ml, for a conductivity of 1.4 to 2.2 mS/cm. Add each bottle separately, stirring as you go.

The pH is adjusted last, once the three parts have dissolved, and it is checked every week in the reservoir. If your system mixes strawberries and herbs, keep a single solution suited to the dominant crop rather than looking for a compromise between very different needs. Every 2 to 3 weeks, empty the reservoir and start again with a fresh solution, because the salts the plants do not take up build up in it. Between two changes, top up evaporated water with clean water. On strawberries, a pH that stays too high for several days shows first on the young leaves, which turn pale between veins that stay green, a sign that the iron has become less available.

The frame assembles square PVC and aluminium tubes of 23 mm using push-fit fittings, then receives the four channels. Allow two people for the assembly, because a 1.85 m channel is awkward to place with one hand and a profile loaded on a single support will twist. The usual guideline for nutrient film is a slope of about 1 cm per metre, which is close to 2 cm of fall between the two ends of a channel: check it with a spirit level before filling with water.

Next, run a test with clear water, pump on, and watch the flow at the end of each channel before installing a single plant. A channel that does not flow along its whole length can be corrected in a few minutes while it is empty. If your ground slopes, shim the feet of the structure before filling the reservoir rather than afterwards, since 64 litres of water make every adjustment heavier.

Feeding your channels with water from a fish tank is a workable practice, on two conditions. The first concerns pH, which must then suit the fish and the nitrifying bacteria first, around neutral, and you lose the freedom to lower it for the plants; the comparison between aquaponics and hydroponics sets out this trade-off in detail. The second concerns suspended solids, which require mechanical filtration upstream, failing which they settle at the bottom of the channels.

On this profile, the height of 80 mm leaves more margin than a shallow channel before a deposit interferes with the passage of the film. This margin does not replace the filter, it only lengthens the interval between two cleanings. Keep the same habit as on a purely mineral installation, and look at the water coming out at the end of the channels each time you walk past. If the flow drops in a single channel, it is its outlet from the supply line that you should clear first.

You receive the four double-walled channels with their removable lids and glued-in end caps, the supporting structure, the 64-litre reservoir, the pump and its supply line, the return circuit and 36 baskets measuring 5 cm in diameter by 7 cm in height. If a channel is ever damaged, the replacement channels in the same profile can be ordered individually or in batches.

The way you pack the pots depends on what you are planting. A bare-root strawberry plant sits in the basket, wedged in place by clay pebbles larger than the slots, which hold the crown above the film without retaining water. A herb cutting or a lettuce seedling is better started in a germination sponge or a rockwool cube, which slips whole into the pot. In every case, the crown stays dry and only the roots reach the film of water, which prevents rot at the base of the stem.